English

Pronunciation

Verb

Oscillation is the repetitive variation,
typically in time, of some
measure about a central value (often a point of equilibrium) or between two
or more different states. Familiar examples include a swinging
pendulum and AC
power. The term vibration is sometimes used
more narrowly to mean a mechanical oscillation but sometimes is
used to be synonymous with "oscillation." Oscillations occur not
only in physical systems but also in biological systems and in human
society.

Simplicity

The simplest mechanical oscillating system is a
mass attached to a linearspring,
subject to no other forces; except for the point of equilibrium,
this system is equivalent to the same one subject to a constant
force such as gravity. Such a system may be
approximated on an air table or ice surface. The system is in an
equilibrium
state when the spring is unstretched. If the system is displaced
from the equilibrium, there is a net restoring force on the mass,
tending to bring it back to equilibrium. However, in moving the
mass back to the equilibrium position, it has acquired momentum which keeps it moving
beyond that position, establishing a new restoring force in the
opposite sense. The time taken for an oscillation to occur is often
referred to as the oscillatory period.

The specific dynamics
of this spring-mass system are described mathematically by the
simple harmonic oscillator and the regular periodic
motion is known as simple
harmonic motion. In the spring-mass system, oscillations occur
because, at the static
equilibrium displacement, the mass has kinetic
energy which is converted into potential
energy stored in the spring at the extremes of its path. The
spring-mass system illustrates some common features of oscillation,
namely the existence of an equilibrium and the presence of a
restoring force which grows stronger the further the system
deviates from equilibrium.

Damped, driven and self-induced oscillations

In real-world systems, the
second law of thermodynamics dictates that there is some
continual and inevitable conversion of energy into the thermal
energy of the environment. Thus, damped oscillations tend to
decay with time unless there is some net source of energy in the
system. The simplest description of this decay process can be
illustrated by the harmonic oscillator. In addition, an oscillating
system may be subject to some external force (often sinusoidal), as when an AC
circuit
is connected to an outside power source. In this case the
oscillation is said to be driven.

Some systems can be excited by energy transfer
from the environment. This transfer typically occurs where systems
are embedded in some fluid
flow. For example, the phenomenon of flutter in aerodynamics occurs when an
arbitrarily small displacement of an aircraftwing (from its equilibrium) results
in an increase in the angle of
attack of the wing on the air
flow and a consequential increase in lift
coefficient, leading to a still greater displacement. At
sufficiently large displacements, the stiffness of the wing
dominates to provide the restoring force that enables an
oscillation.

Coupled oscillations

The harmonic oscillator and the systems it models
have a single
degree of freedom. More complicated systems have more degrees
of freedom, for example two masses and three springs (each mass
being attached to fixed points and to each other). In such cases,
the behavior of each variable influences that of the others. This
leads to a coupling of the oscillations of the individual degrees
of freedom. For example, two pendulum clocks mounted on a common
wall will tend to synchronise. The apparent motions of the
individual oscillations typically appears very complicated but a
more economic, computationally simpler and conceptually deeper
description is given by resolving the motion into normal
modes.

Continuous systems - waves

As the number of degrees of freedom becomes
arbitrarily large, a system approaches continuity; examples include a
string or the surface of a body of water. Such systems have (in the
classical
limit) an infinite
number of normal modes and their oscillations occur in the form of
waves that can
characteristically propagate.